Radioactive waste has arisen from two major sources: production of nuclear weapons and production of nuclear energy. The waste can take at least three forms. By far the largest volume is liquid waste from commercial nuclear energy generating plants. To recover unused uranium and/or plutonium, the spent fuel rods are dissolved in nitric acid. After removal of these actinides, the strong acid wastes are neutralized and stored in steel tanks. The problem has been that the tanks corrode with subsequent leakage of high-level radioactive liquids into the biosphere.
One can convert the radioactive liquids to solid oxides but this physical form can also be dispersed fairly easily. These powders are generally referred to as calcines. The level of radioactivity from calcine is very high and of the order of 1.5 million rads (R) per hour as a dosage. After storage for a hundred years, the level will have dropped to 5800 R per hour but 1000 years storage is indicated before an acceptable dose-rate for humans arises. However, the above refers only to sub-uranic, or fission product wastes. If the actinides such as uranium and plutonium are not removed, then the wastes must be kept in secure storage for about 250,000 years before they can be considered safe for human exposure.
The volume of commercial waste (high level waste--HLW) is enormous. About 74 million gallons have existed, or will exist once the stored spend fuel rods are processed. Because of the lack of a really satisfactory disposal method for HLW, a major part of the spent fuel rods have been stored under water in underground bunkers. The United States has sufficient uranium stockpiled so that recovery of unused uranium from the spent fuel rods is not critical. However, this practice cannot continue indefinitely. Some of the liquid waste already produced has been converted to calcine. There is about 3.9 million (M) cubic feet of unprocessed liquid waste which will form some 585,000 cubic feet of calcine.
The second form of radioactive waste consists of actinide waste which has been separated from HLW and other sources. It amounts to about 1.8 M cubic feet of liquid waste. The third form of radioactive waste, weapons waste, amounts to about 75 M gallons, or about 9.6 M cubic feet. This waste is of lower radioactivity level than that of HLW from reprocessing of commercial fuel rods, which in turn is much less than that of separated actinide waste, as regards radioactive emissions level.
The use of glass for containment of high-level radioactive waste has been under development for many years. There are many attractive features of this mode of encapsulation. They include a rigid incorporation of the radioactive ions, or species, by dissolving them into the melt to form the glass structure. They are then not free to move as long as the glass structure is maintained. Glass is not subject to grain growth, surface oxidation, and other factors common to crystalline solids. However, there are six critical properties required for any glass in this application. These include: (1) minimal tendency to devitrify, (2) low hydrolytic leach rate, (3) high solvency power, (4) relatively low melt temperatures, (5) low tendency to form crystals from the added waste components, and (6) low softening point and viscosity of the melt.
Devitrification refers to the proclivity of an amorphous solid (glass) to become crystalline. All glass will devitrify provided that the internal temperature of the glass body is raised to a certain point called the devitrification temperature. The devitrification process is exothermic; that is, it releases heat, so that when devitrification starts, it is self-sustaining. The devitrification product consists of microcrystals so that the mass is friable and easily dispersed. It is therefore important to maintain the amorphous state for the HLW encapsulation application. The problem is that the incorporated HLW is a heat source through natural fission processes plus absorption of energy from the emitted radiation by the glass matrix. Internal temperatures of up to 850.degree. C. have been observed. Thus all of the prior glasses used for this application have devitrified when the incorporated HLW has heated the glass to its devitrification temperature during storage. This remains a severe problem for which there has been no solution heretofore.
Since the HLW-glass is to be stored for prolonged times as a solid mass, the hydrolytic leach rate, as a loss at the surface of the glass body, is important. Ordinary window glass has a relatively high leach rate of 5.3.times.10.sup.-4 gm/cm.sup.2 /hr in boiling water. A good waste-glass must have a value of at least 150 times smaller than this. Granite, an igneous rock, has a leach rate of about 4.6.times.10.sup.-6 gm/cm.sup.2 /hr while that of marble is about 1.2.times.10.sup.-5 gm/cm.sup.2 /hr. Since the waste-glass is to be stored in underground rock vaults, its hydrolytic leach rate ought to be less than the surrounding rock.
When the HLW is to be added to the glass melt, all of the components need to be dissolved. Many of them are refractory oxides such as CeO.sub.2, ZrO.sub.2 and RuO.sub.2. A high solvency power of the melt is therefore needed. In most glasses, the addition of excess oxides to the glass melt tends to cause formation of insoluble crystallites as specific compounds which begin to recrystallize and grow larger. When the melt is cast, the crystals, as a second phase, form centers of internal strain, thereby causing the glass to develop cracks and become friable. Hence it is also desirable if the glass exhibits little or no tendency for internal crystallite formation.
Furthermore, the processing temperatures required for production of glass need to be relatively low for nuclear waste encapsulation, preferably not over 1400.degree. C. Conservation of energy is one reason for this limitation while another is that the containers intended for actual storage of the waste-glass cannot stand processing temperatures in excess of this value. Finally, the glass melt also needs to have a low viscosity so that added waste oxides can be dispersed into the melt more easily.
The best glass known heretofore for the nuclear waste encapsulation application, a zinc borosilicate (ZBS), was developed especially for this purpose. A melt is produced at 1400.degree. C. which has a viscosity of less than 200 poise. Up to 45% by weight of the HLW oxides can be dissolved into the melt. The hydrolytic leach rate is lower by an order of magnitude than most commercial glasses. Unfortunately, HLW-ZBS glass devitrifies at 750.degree. C. and softens at 570.degree. C. Refractory waste oxides such as RuO.sub.2, CeO.sub.2 and ZrO.sub.2 do not dissolve at all well into the melt and crystallites of Zn.sub.2 SiO.sub.4, SrMoO.sub.4, NdBSiO.sub.5 and Gd.sub.2 Ti.sub.2 O.sub.7 are among the crystalline compounds observed to form in the glass or devitrified product.